Hi Jon.
I’ll give you my opinion on them, but you should still research them more:
The pro-forms are nice, because you can get payments on your site, and that’s with PayPal Pro or Authorize.Net. You’ll need and SSL certificate for that.
The buttons will have the user go to the gateway’s site to checkout. It’s nice that they deal with the security and customers may feel good with that too, since they’d recognize the brand (PayPal, ClickBank, etc).
There’s also the PayPal pro-form without PayPal Pro, so it’d use Express Checkout, which lets you have the pro-form, but the user still goes to PayPal’s website to pay. And there’s PayPal Standard, which is the free service with buttons.
ClickBank accepts cards, but also PayPal, so that’s good. And it has a built-in affiliate program, they have a ton of affiliates that could promote your product if you have a good offer. ClickBank does require you to give a 60 days money-back guarantee.
I have no experience with Google Checkout. ccBill is for adult sites, so it’s more expensive and would not be worth the trouble if you’re not in that industry.
I hope that helps. :)